1. Field of the Invention
The present invention relates to a developing unit that is used in an image forming apparatus, such as a copying machine, a printer or a facsimile machine, and more particularly to a developing unit which is capable of optically detecting the residual quantity of a developer in a container.
2. Description of the Related Art
Conventional image forming apparatuses employing an electrophotographic recording method, such as page printers and facsimile machines, usually comprise a unit for detecting the residual quantity of a developer in order to issue an alarm to a user prior to reduction in the developer in a developer container an amount that causes a thin and blurred image to be formed. The unit detects the residual quantity of the developer in the container by measuring any one of (1) the weight, (2) electrostatic capacity, (3) change in the torque of a stirring member, (4) the quantity of transmitted light or the like. The method (4) is widely employed because the apparatus can be constituted simply; the cost can be reduced; and the detection accuracy is relatively satisfactory. The foregoing method is characterized in that a light emitting device and a light receiving device are provided for the image forming apparatus body, and transparent windows through which light emitted by the light emitting device passes are formed in the developer container so that change in the output from the light receiving device occurring due to reduction in the developer in the developer container is used to detect the quantity of the residual developer. If the developer adheres to the surfaces of the transparent windows, light cannot sufficiently pass through the transparent windows, and therefore the detection accuracy deteriorates. Accordingly, a wiping member for removing the developer on the surfaces of the transparent windows is disposed in the developer container. In general, the wiping member is a flexible sheet member that is disposed together with a stirring member for stirring and conveying the developer in the developer container. The wiping member is rotated so as to be brought into contact with the surfaces of the transparent windows to remove the developer adhered to the surfaces of the transparent windows. Thus, the residual quantity of the developer can be detected stably and accurately.
In the foregoing case in which the wiping member is provided for the stirring member, the developer is forced to fly by the stirring member and the wiping member to thereby again adhere to the transparent windows.
Recently, there is a desire for reducing the size of the image forming apparatus, suck as a copying machine, a page printer, a facsimile apparatus or the like. Such a desire leads to a fact that the sizes of a developing unit and a process cartridge that are included in the foregoing apparatus must be reduced. Also the capacity of the developer container must be reduced. However, the conventional method has a critical problem when the quantity of the residual developer is detected in a light transmissive manner. In order to accurately detect the residual quantity, the transparent windows must be disposed adjacent to the developer carrier. Use of the conventional rotative wiping member causes the wiping member to be easily brought into contact with the developer carrier. Therefore, the transparent windows must be disposed away from the developer carrier and therefore the residual quantity cannot be detected accurately. In addition, further use of the developing unit or the process cartridge is inhibited though a large quantity of the developer is left in the developer container. Thus, an economical problem arises.
If the contact pressure of the wiping member with the transparent windows is inadequate, the force of the wiping member to slide on the surfaces of the transparent windows is weakened. Thus, the developer is left on the surfaces of the transparent windows and a predetermined output voltage cannot be obtained from received light. Therefore, there arises a problem in that an image thinner than a desired density is formed prior to issuing an alarm of a wanting state of the developer to a user. Specifically, the conventional developing unit comprises a flexible sheet serving as the wiping member and made of PET (Polyethylene Terephthalate) or urethane rubber that has a problem in that it is permanently deformed if it is left for a long time while being deformed. If the operation of the developing unit is completed in a state where the wiping member is in contact with the transparent windows or the developer container and the foregoing state is maintained for a long time, the wiping member is deformed permanently. Such deformation leads to a fact that the contact pressure of the wiping member with the transparent windows is lowered excessively and the transparent windows cannot be cleaned satisfactorily. Thus, a satisfactory quantity of light passing through the transparent windows cannot be obtained. The foregoing phenomenon becomes critical at high temperature and high humidity.
What is worse, a small size developing unit or process cartridge encounters a problem of the image quality because the torque may be enlarged or changed due to the presence of the wiping member.